Physical Science
Ch. 2: Motion
Motion
Motion is the change in
position of an object.
A reference point is
usually needed in order
to determine motion or
rate of motion.
SR-71 Blackbird
Examples:
• SR-71 Blackbird
• Tossing an object in a moving bus
• Earth’s rotation
• Virtual reality rides
• Car sickness
• Displacement is a change in location of an
object (distance and direction from starting
point).
• Is displacement the same as motion?
Hmmm…
No, not necessarily.
A spinning basketball for example, is in
motion. But it’s location does not have to
be changing, so it may not be displaced.
Speed
While motion refers to the change of position
of an object, speed describes the rate of
change of position, and velocity describes
both speed and direction.
The SI unit for velocity is m/s.
Instantaneous speed is the speed at 1
given moment in time.
Constant speed is speed which neither
increases or decreases.
• Average speed is calculated by dividing
distance by time.
V = D/T
Try the following problem.......
Cletus drives from Little Rock to
Texarkana, a distance of 150 miles.
If his average speed was 60 mi/hr,
how long did the trip take?
time = distance / velocity
150 miles / 60 mi/hr = 2.5 hrs. or
2 hours and 30 minutes.
• Let’s say you go to watch the fireworks on
the 4th. From where you’re sitting, you are
501 m away when a skyrocket explodes.
If you hear the explosion 1.5 s later, what
is the speed of sound (m/s)?
• When a plane breaks the sound barrier (faster than
the speed of sound, 334 m/s) it produces a loud
shock wave called a sonic boom.
• Suppose an F-18 flying overhead breaks the barrier,
and you hear the sonic boom 12 sec. after it was
produced. How high in the air was the jet flying?
Acceleration
Acceleration is the rate of change of
velocity (how fast something speeds up
or slows down) and is measured in
meters/second/second (m/s/s or m/s2)
For example, if a car speeds up by
20 meters per second every second,
than the rate of acceleration is 20 m/s/s.
If the same car continued to accelerate
at 20 m/s/s for 5 sec., it will increase it's
velocity by 100 m/s.
Start – 0 m/s
1 sec. – 20 m/s
2 sec. – 40 m/s
3 sec. – 60 m/s
4 sec. – 80 m/s
5 sec. – 100 m/s
the formula for acceleration is:
A = (Vf - Vi) / t
Vf - final velocity (speed it ends up at)
Vi - initial velocity (speed it started at)
T - time
Now try this practice problem.
The Thrust SSC is the only vehicle to ever break
the sound barrier on land, going 760 mi/hr.
If the vehicle went from 0 to 760 mi/hr in 40 sec.,
what was it’s rate of acceleration in mi/hr/sec?
A = (Vf - Vi) / t
Vf = 760 mi/hr
Vi = 0 mi/hr
t = 40 sec.
A = (760 – 0) / 40
A = 19 mi/hr/sec.
The rate of acceleration of an object can
also be a negative value.
Negative acceleration simply means that
the object is slowing down.
Forces
A force is any push or pull exerted on an
object.
As you sit here, there are at least 3 forces
currently acting on your body.
What are they? Hmmm.
1. Gravity
2. Supporting force
3. Air pressure
Balance Forces
• Forces which are equal in size and opposite
in direction are said to be balanced forces.
• Ex.: -chair and gravity
-tug-o-war (tied
-2 people leaning back-to-back
Unbalanced Forces
Unbalanced or net forces
are not equal in size. And
since this means that the
force in 1 direction must
be greater, it will result in
a change in velocity.
Ex.: - kicking a ball
- ears popping
- pushing the desk
Newton's First Law of Motion
• Newton's 1st LOM
states that an object will
maintain a constant
velocity unless acted on
by a net force.
This includes both
stationary and moving
objects.
• Newton's 1st law is also
called the Law of Inertia.
Inertia is the tendency of
an object to resist any
change in motion.
Inertia is related to the
mass of an object. What
is this relationship?
Motion and Forces
What happens in a crash?
• The law of inertia can explain what happens
in a car crash.
• When a car traveling
about 50 km/h
collides head-on with
something solid, the
car crumples, slows
down, and stops
within approximately
0.1 s.
Motion and Forces
2.3
What happens in a crash?
• Any passenger not wearing a safety belt
continues to move forward at the same speed
the car was traveling.
• Within about 0.02 s (1/50 of a second) after
the car stops, unbelted passengers slam into
the dashboard, steering wheel, windshield, or
the backs of the front seats.
Gravity
• Gravity is a force of mutual
attraction which every
object in the universe
exerts on every other
object.
The amount of gravity
depends on the distance
between the objects and
the mass of the objects.
• What is the relationship between the
following:
–Gravity and distance?
–Gravity and mass?
• A black hole is a collapsed star with so
much gravitational pull that even light can
not escape.
Does a cat always land on it’s feet?
Weight
• Weight is a measure of the gravitational
pull exerted on an object.
• The SI unit for weight is the newton (N).
1 newton is about the weight of a medium
sized apple. On earth, 1 N is just about
.25 pounds.
• Weight is measured using a scale.
“Don't step on it ... it makes you cry.”
Topics for Discussion
• Special effects
• Falling elevator
• Tides
Friction
• Friction is a force which opposes
motion between two surfaces which
are in direct contact.
• There are 2 main variables which help determine
the amount of friction present between 2
surfaces:
1. The force pushing the surfaces
together
2. The texture of the surfaces
Topics for Discussion
• Hydroplaning
• Swamp buggies
• Starting fires
• Most meteors burn up as they are passing
through the earths atmosphere, yet the
space shuttle which re-enters the
atmosphere at a high speed is relatively
unharmed. Why do you think this is?
Calculate the velocity of the car shown above.
• Viewing the cars above, one is travelling at a
constant velocity, one is experiencing positive
acceleration, and one is experiencing both
positive and negative acceleration. Which is
which?
• The illustrations
show an elephant
and a feather both
falling from the top
of a building at the
exact same time.
Explain what could
cause the
difference between
the two illustrations.
• How are each of the 3 objects in the above
illustration accelerating?
• A robotic explorer was sent to planet
BR-549. The rate of gravitational acceleration
on this planet is 29.4 m/s/s. If the explorer
weighed 2,500 pounds on the earth, what was
it’s weight on this other planet?
• Is BR-549 larger or smaller than the earth?
• For the skydiver in the
illustration, tell what is
happening to his acceleration
and velocity during the
following stages of his trip:
A. Initial falling stage
B. Time when the parachute is
being deployed
C. Falling stage with the open
parachute
• In the above scenario, a small cannon shoots an
iron ball straight up in the air, from the back of a
truck traveling at a constant velocity
• Is the above situation possible in real life?
Explain.
• Some people may put a tube of sand in
their trunk or in the bed of their truck.
In terms of something that we’ve discussed
in this chapter, explain why they would do
that.
• Before a drag race, the dragster will often “burn
out” at the starting line before the race occurs, and
then immediately back up to the line and get set for
the start of the race.
Why do you think they do this?
• Some good pitchers can throw a fastball which
actually rises from the time it leaves their hand until
the time it reaches the catcher. How do you think
this is possible when gravity is pulling it downward
the whole way?
• Roy has to slam on his brakes when a
deer runs in front of his car. If he was
going 72 mi/hr and it took him 6 seconds
to stop, what was his rate of acceleration?
• Rob drives from Kansas City to St. Louis
(210 mi.) traveling at 70 mi/hr. From St.
Louis he turns north and drives to Chicago
(270 mi.) averaging 60 mi/hr. How long
did it take him to make the entire trip?
• A 400 m runner
accelerates at a rate of
.5 m/s/s over the last 6
seconds of the race, in
an attempt catch an
opponent at the finish
line.
If she ends up running
at 12 m/s, how fast
was she running
before she started her
kick.
Jim is driving from Kansas
City to Denver. It takes him
7.5 hrs. to drive through
Kansas at an average
speed of 70 mi/hr. He then
drives for 2 hrs. and 30
min. in Colorado at an
average velocity of 60
mi/hr., before arriving in
Denver.
How far is it from K.C. to
Denver?
At or between which point(s) on the above
speed/time graph is the object experiencing:
A. Constant speed
B. The greatest negative acceleration
C. Top speed
D. The most gradual positive acceleration